The present invention relates to the field of mobile communication between a base station and a mobile terminal, and in particular to connections with high transmission rates in a Time Division Multiplex Access (TDMA) system, such as the GSM system.
In a TDMA system, communication between the base station and a mobile terminal takes place in channels. A number of channels are transmitted on one carrier frequency using time division multiplex. The transmission on each carrier frequency takes place in time slots, and each physical channel occupies one time slot. As an example, in GSM, eight physical channels at a time may share the same carrier frequency, that is, eight time slots constitute one frame. One traffic channel (TCH) occupies one physical channel, and one connection usually involves one TCH. In this way, all connections are transferred at the same maximum bit rate. At present, the maximum bit rate is 9.6 kbit/s for payload information.
According to the GSM standard, frequency hopping may be used, that is, at regular intervals the carrier frequency is changed, to minimize the effects of multipath fading and the disturbances between the channels.
High Speed Circuit Switched Data (HSCSD) has been introduced in the GSM standard to enable connections with higher transmission rates. HSCSD connections use a multislot configuration of channels for transmitting data, that is, one connection is allowed to occupy more than one channel, that is, more than one time slot in each frame. The network architecture to support HSCSD allows a maximum of eight independent full rate traffic channels to be used for one connection, to achieve a bit rate eight times higher than the normal bit rate.
Different mobile terminals are capable of handling different numbers of channels. The maximum number of uplink channels and downlink channels, respectively, and the total maximum number of channels may be restricted. In the GSM standard, 18 different mobile classes have been defined, specifying the number of channels that a mobile terminal is able to handle. Other restrictions are also specified; for example some mobile terminals can only handle consecutive channels in a multislot connection, whereas others can handle any combination of channels. The simplest mobile class only handles one uplink channel and one downlink channel. The most advanced mobile class handles up to eight channels in each direction, and any combination of channels may be used.
The GSM standard states that all channels to be allocated to one mobile connection in a multislot configuration must have the same Training Sequence Code (TSC), Hopping Sequence Number (HSN), Mobile Allocation (MA) and Mobile Allocation Index Offset (MAIO), if frequency hopping is used. If frequency hopping is not used, all channels used in the same multislot configuration must have the same TSC and Absolute Radio Frequency Channel (ARFCN). This means that all channels in a multislot configuration transmit and receive on the same frequencies at the same time, with the same TSC, even when frequency hopping is used.
According to the GSM standard, different speech versions may be used, in dependence of the type of mobile terminal used. Version I full rate was the original speech version and is still used by some mobile terminals. Later, Version II, enhanced full rate and half rate have been added. Different channels support different speech version. One channel may support only one speech version or several different ones, depending on the network equipment used.
For standard connections occupying one channel, algorithms have been disclosed for allocating one channel to one connection. For example, U.S. Pat. No. 5,448,750 discloses a method for dynamic channel allocation. Channels are ranked in a priority list according to their performance in previous connections.
There is, however, no way of allocating more than one channel to a connection in a suitable way. If a connection requires the use of more than one channel, normally a known algorithm for assigning one channel is used. If there are other idle channels on fulfilling the requirements for a multislot connection, that is, transmitting on the same frequency and with the same frequency hopping pattern, these channels may then be used for a multislot connection. Depending on the mobile terminal used, the channels used in a multislot connection may have to occupy consecutive time slots in a frame, which also cannot be ensured with known methods. Thus, there is virtually no way of ensuring that the desired number of additional channels can be added.
In most mobile telephony systems, channel spacing is used, that is, the uplink carrier frequency and the downlink frequency used in a connection are always at a specified distance from each other. When a carrier frequency has been selected for one direction, the carrier frequency to be used in the other direction is given. Thus, when high transmission rates are desired in both directions, it must be ensured that the desired number of channels are available on both the uplink and the downlink carrier frequencies.
Ideally the allocation of multislot connections should fulfil the requirements listed below:
The maximum radio interface data rate should be as high as possible.
The idle channels should be selected in such a way that the chance of establishing subsequent multislot channels is maximized.
The idle channels should be selected in such a way as to allow allocation of suitable channels to subsequently requested speech connections. Therefore, the channels supporting the largest number of speech versions should be left idle if possible.
The idle channels should be selected according to the strategy preferred by the operator regarding non-hopping TCHs on the BCCH frequency.
Channels with low interference should be selected.
The channels should be selected in such a way as to minimize the impact of interference and multipath fading in the system, by using frequency hopping in an efficient way.
The channel allocation algorithm should be fast.
The channels should be selected in such a way as to allow the desired number of channels in the uplink direction and the downlink direction.
It is an object of the present invention to define a way to select and allocate channels for multislot connections of the desired capacity.
It is another object of the invention to allocate channels to multislot connections in such a way as to minimize the interference between the connections.
It is yet another object to decrease the time needed for channel allocation in multislot configurations compared to the prior art solutions.
It is still another object to achieve a method fulfilling the requirements listed above.
These objects are achieved according to the present invention by a method for the allocation of channels to a high transmission rate connection between a base station and a mobile terminal in a mobile telecommunication network, said high transmission rate connection being achieved by allowing one connection to occupy more than one physical channel, said method comprising the following steps:
Sorting all channels that may be used for transmitting traffic in the mobile network into groups, each group comprising all channels that are allocatable to one connection in the network;
When requesting a connection, specifying a number of channels desired for the connection;
Selecting a suitable group for the connection.
To refine the selection, the method may comprise the following additional steps:
identifying the mobile class of the mobile terminal that is to take part in the connection.
identifier the groups that have at least the desired number of idle channels.
if no groups have at least the desired number of idle channels, identifying the groups having the highest number of idle channels.
identifying the groups having the lowest number of idle channels above the desired number of channels.
identifying the groups from the least adaptable TCH capabilities group.
identifying the groups having the lowest interference level.
identifying the groups following the selected strategy for assigning the traffic channels on the BCCH frequency.
identifying the groups hopping over the largest number of frequencies.
The method according to the invention may be performed for downlink channels, uplink channels or both.
A network allocation node for use in a mobile telecommunication network is also disclosed, said node comprising:
a list of all groups of channels that can be used together for a connection between a base station and a mobile terminal,
means for carrying out the inventive method.
The invention offers the following advantages:
The highest possible total radio interface transmission rate is achieved for each connection set up according to the invention.
The probability of success in achieving high transmission rates for multislot channel allocations is increased compared to the prior art solutions.
The probability of allocating channels supporting the requested speech version for further speech connections is increased compared to the prior art solutions.
The idle channels may be selected to minimize multipath fading and achieve interference averaging when multislot configurations are allocated, giving an improved and more uniform speech quality and enabling a shorter frequency reuse distance.